A Useful Site for Teaching College Physics: The Physics Source

Today I would like to feature this website known as the "The Physics Source".  It is a featured by Compadre which is also another site which contains very useful resources for students, teachers and faculty members.  However Compadre is a very large site that contains many, many, many other references to sites, so I will rather review each one as I come across.

I particularly like this site as it contains many materials particularly suited for teaching introductory College Physics.  As there are many topics that are also taught in A-levels, the materials can be easily adapted for a A-level classroom.

All the resources are already classified thematically e.g. mechanics.  Under each theme the resources are further classified topically.  Once you enter into the topics, you will have a view of the available resources.  Beyond the title of the resource, there is a brief description on what each resource is and also the weblink to get the resource.

This will definitely be a website I will be visiting see what are the resources available for the topics I am teaching or lecturing.  This is so exciting!

Waves in the Sky - Wave Clouds

[Picture Source:  Compliments of Mr Ong Jek Yaw, 2010]

In the teaching of waves and standing waves, we often use the ripple tank to illustrate phenomenon associated with waves.  Today, I learnt something from a very experienced colleague of mine (Mr Ong Jek Yaw).  In reality, we have an extra large "ripple tank" in our sky!  The rows and rows of clouds that form the sky are in reality known as wave clouds, and they are formed when the atmosphere is disturbed and a "gravity" wave travels through.

From what I understand, wave clouds are form when a thin and stable layer atmosphere is disturbed, e.g. when it passes over a mountain.  The air disturbance can propagate like ripples in the pond when the stone hits the water.  As the wave moves through the air, it undergoes repeated uplift and descent. If there is enough moisture in the atmosphere, clouds will form at the crests of these waves. In the descending part of the wave this cloud will evaporate due to adiabatic heating, leading to the characteristic repeating cloud/clear bands. (wikipedia)

Stationary waves form in the atmosphere can also result in structures observed as well.

Two particularly interesting wave clouds video can be found on youtube and are embedded below:



[ Video above from Youtube, by akrherz at http://www.youtube.com/watch?v=yXnkzeCU3bE]

[Video above from Youtube by RobyF5  at http://www.youtube.com/watch?v=Dvl3PrXwBkI&feature=player_embedded]
More about wave clouds can be found in the links below.

http://www.weatherquestions.com/What_causes_wave_clouds.htm

http://www.theweatherprediction.com/habyhints/64/

http://en.wikipedia.org/wiki/Wave_cloud

http://weathervortex.com/sky-ribbons.htm

Winplot - My Favourite Graphing Software! It is FREE!

In the teaching of physics, it is often that we need to do plots of equations. Many A-level teachers tend to use Excel. This is great when I am creating questions which requires students to take points off graph grids, however, not so great when I just want general (but accurate) shapes to different graphs.

My search of a software became imperative when I was lecturing waves and wanted to generate many different sinusoidal graph starting at different phases. After plotting a few graphs on excel, I gave up and decided to ask a (Further) Math teacher, who told me that she used a free software Winplot.

I have since been using Winplot to generate all my graphs. I found it relatively easy to use for generation of graphs - just type in the equation and the software auto generates for you. You get the flexibilty to change the scale set the thickness and it can be easily copied to the clipboard which can be pasted in any Word document.

It has been almost 7 years since I found Winplot and am still using it. The author constantly makes improvements to it and it is now one of the most powerful graphing software and the great thing is it is still FREE! It is also relatively easy to pick up and there are many external "instructors" that have produced tutorials, manuals, and video instructions.

If you are looking for a free graphing software, this is definitely one to try out.

Most updated version can be found at : http://math.exeter.edu/rparris/winplot.html

A good startoff point for learning are a set of instructional video by Steve Simonds which can be found here : http://spot.pcc.edu/~ssimonds/winplot/

Other supplementary materials can also be accessed here: http://math.exeter.edu/rparris/wpsupp.html

There are some youtube videos as well, one of which is given below.  If you like them, you can google for more at youtube.

A Resource for Teaching Semiconductors: The Solar Cell

An Introduction to Solar Cells

Phys. Teach. 48, 306 (2010)

http://link.aip.org/link/?PHTEAH/48/306/1

[Picture from: http://www.sxc.hu/photo/1054612, johnnyberg, The essens of summer]

The study of Semiconductors and explanation of its behaviour by band theory was introduced into the H2 Physics Syllabus in Singapore for A-level Physics in 2006. Generally, for this topic we learn the mechanics of it, but so far there has been little exploration of its applications in real life.

The May 2010 issue of the Physics Teacher (see link below) carries an interesting article on how solar cells work. It explains the workings of it using band theory as it is a semiconductor. It should find great relevance in today with environmental issues becoming a pressing problem that the world faces.

The link is listed below. I would like to explore how to further apply this article to classroom teaching and integrate this into the A-level syllabus, either as a reading for an activity that can enthuse students or even as an examination question.
The abstract is quoted below:

Abstract : (©2010 American Association of Physics Teachers)

"Most likely, solar cells will play a significant role in this country's strategy to address the two interrelated issues of global warming and dependence on imported oil. The purpose of this paper is to present an explanation of how solar cells work at an introductory high school, college, or university physics course level. The treatment presented here will be qualitative and somewhat simplified, in order to reach the desired audience; references are provided for a more detailed and mathematically sophisticated treatment. It is hoped that this paper will, in a small way, motivate students to learn more about this technology, so critical to the energy and environmental future of this country. "

Why a smaller sphere having a smaller charge can have an electric field that is stronger than a larger sphere having a larger charge?

This is a followup post to my previous post on the misconception that a smaller spherical conductor will have a larger charge and hence there is a greater possibility of it being discharging.

I did the same question with another of my class today.  However, this time, this class of students had more problems in visualising why a smaller sphere having a smaller charge could possibily have a stronger electric field at the surface.  I also had students who told me that their secondary school teachers told them that it was the charge per unit area that mattered and asked me how to reconcile this with the concept of electric field being stronger.

Let us address the first issue first.  To aid in the visualisation.  I drew the following picture.  In this case, we had already made a calculation of 0.3 micro-coulomb on the small sphere and 0.9 micro-coulomb residing on the larger sphere.  So if there is one electric field line drawn for each 0.1 micro-coulomb charge on it sphere, then we see can have:

So from the diagram above, we see that the although there is less charge on the smaller sphere, the electric field lines could be closer, and hence the E-field could be stronger.

Now for the second question, was the secondary teacher right in saying that it is the charge per unit area that mattered.  Two ways to understand this, first the diagram shows fundamentally an essential point in drawing field lines, the no. of field lines are proportional to the charge, therefore have a greater no. of charge per unit area, essentially means more electric field lines per unit area and hence the field lines will be closer and in other word, the electric field is stronger.

Alternatively, those who prefer to see equations will have 

From the equation, we can see that the electric field strength at the surface of the charged sphere can be written as a constant multiplied by  (Q / surface area of sphere), so essentially the secondary teacher who relates the electric field at the surface to the Q per unit area is actually correct.

A Common Misconception: More Charges Accumulates on a Sharper Point...Is it Really True?

[Picture Source: stock.xchng, "Raw Power1" (2008) by gun4fire]

When I was an O-level student, I was often told by my Physics teacher that as more charges tend to accumulate on a point that is sharper and hence there is a greater likelyhood for discharge and therefore, lightning rod are built to be sharper.  As a typical O-level student, I was like my student now, and just readily accepted what my teacher said then.

While teaching electric field at A-levels, we have a typical problem whereby we have two spherical conductors in which we connect a "long" wire across them.  We then place a certain amount of charge onto the system and the charges with redistribute till they come to electrostatic equilibrium.  (A similar problem can be found in Serway's "Physics for Scientists and Engineers"(6th Edition), Problem 25.50, pg 791.)

After the calculation, we may find some interesting results.  The electrical charges collected on the larger sphere are in reality more than that of the smaller sphere.  However the electric field at the surface of the smaller sphere is large.  The electric field represents the force acting on per unit charge, and hence logically the larger the force the higher the probability of discharge.  Hence I guess, the idea of more charges accumulating at a sharper point and hence greater probability of discharging, can be quite misleading....


PS:  This article is a followup on a previous comment contributed which triggered this idea.  It is possible to start off with the Windhurst machine activity and then follow up with the problem above and a discussion of breakdown voltage.

Problem 25.50 in Serway (6th Edition), pg 791:
Electric charge can accumulate on an airplane in flight.  You may have observed needle-shaped metal extensions on the wing tips and tail of an airplane.  Their purpose is to allow charge to leak off before much of it accumulates.  The electric field around the needle is much larger than the field around the body of the airplane, and can become large enough to produce dielectric breakdown of the air, discharging the airplane.  To model this process, assume that two charged spherical conductors are connected by a long conducting wire, and a charge of 1.20 micro-coulombs is placed on the combination.  One sphere, representing the body of the airplane, has a radius of 6.00 cm, and the other, representing the tip of the needle, has a radius of 2.00 cm.  (a)  What is the electric potential of each sphere?  (b)  What is the electric field at the surface of each sphere?

Popular Science Archive is now free!

My husband and I have always enjoyed Popular Science magazine, and used to be a subscriber till my magazine vendor stop carrying it. It is a great magazine targetted for the layman as it introduces in plain language what are some of the technologies that people are recently developing around the world. Occasionally, I can come across interesting reads that I can use in my lessons.

Good news for all! Recently, Popular Science are partnering with Google to make available their 137 years of archival magazines Popular Science and Popular Mechanics. You can find the archive at http://www.popsci.com/archives.

Or alternatively, you can get both Popular Science and Popular Mechanics in Google Books! I hope this is a trend for magazines to come.

A Greater Need to Build the Hands On Skills of Our Children

I had a nice gathering with a bunch of present and ex-colleagues of mainly Physics teachers and was discussing about the practical skills of our present day students. We all shared the same sentiments, the hands-on skills of our students have declined over the years.  We had stories of students who were not able to strike matches, students who did not know how to tie a simple pendulum - they said that they never tied shoelaces as their track shoes came with shoe laces tied and their parents or maids would tie it back for them after the shoes are washed.  There were students who could not handle drills, and almost drilling their hands when by using the hand to hold a wooden stick directly below the hole to drilled.  Students who also had problem using saws and hammer and would be terrified when given the tools.

These are the group of students, who spent their childhood at home playing with toys, computers or drilling on homework.  They had a different childhood from many of us teachers,who did not have access to computers which only became a common household item when we were in university or when we were working.  We had spent our childhood playing with neighbours, running around in the neighbourhood, building structures to have war games (though I was a girl, I was always following my brother and engaging in the activities he was involved in).  During mid autumn festival, we had lanterns or some made lanterns from pomelos peels, tin cans which we lighted candles in. For me and my friends, we also spend quite alot of our teenage years, fishing, camping and also staking out in abandoned kampungs with durian trees to wait for durians.  Alas, what a childhood we used to have.  It is no wonder, the kids do not have the hands on ability to build or handle apparatus in Physics experiments.

I shall resolve that my children will not be like that and I feel that the hands-on practical work should have a greater emphasis not only in the Secondary and Pre-university school years but also starting at a very young age.  Children should be exposed to various activities that require them handle tools, build things.  Performing the experiment and getting them to just infer or deduce results, which many schools are doing are just not sufficient.  You must literally do the work, get "dirty", handle the tools to acquire the dexterity for more complex work later.

Useful Tool for Drawing 3D - Google sketchup

I was at the Times bookstore last Saturday as it was my birthday month and I wanted to spend away 20% discount card.  I came across a new free tool called Google Sketchup.  It is tool that allows for drawing of 3D diagrams. 

I think that it will be a useful tool for Science Teachers.  If you do a google on sketchup, you can also find many free models for sciences available through the internet.  I have not learnt how to use it yet, but will definitely get to loading and using it.

The tool is available for download at http://sketchup.google.com/